 It's actually a barrier. And when we have allergies, it's a protective barrier that tries to block or coat the allergies from entering. And the body reacts by producing that excess fluid. And it sometimes drips in the back of our throats. We get what's called post-nasal drip. And then we want to spit it out. Yes, if it stays in your sinuses for a while or you get a lot of fluids and it stays stagnant, then we end up with a sinus infection.